The invention relates to the art of pressure molding ultra high molecular weight polyethylene (UHMWPE) to molecularly bond the UHMWPE to an article, such as a bolt, screw, plate, and reinforcing member.
Ultra high molecular weight polyethylene, herein identified as UHMWPE, is a thermoplastic having advantageous characteristics including high abrasion resistance, low coefficient of friction and exceptional toughness. UHMWPE is used in a number of applications in various industries which require abrasion resistant and low coefficient parts. The average molecular weight of UHMWPE is from 4 to 5 million. The plastic polymer is made up of long branch free chains, and when compression molded, gives a material of impact strengths at room temperature greater than other thermoplastics. The structure of UHMWPE makes it highly resistant to repeated stress, impact and abrasion. The use of UHMWPE is limited due to difficulties in its processing. The high melt viscosity of UHMWPE limits its processing to presses and screw or ram extruders. The formed pieces are machined to desired shapes. Examples of processes for molding UHMWPE are disclosed in U.S. Pat. Nos. 2,400,094; 3,847,888; 3,975,481 and 5,453,234.
The invention comprises a method for making solid articles by and molecularly bonding to metal and other materials and the completed articles, such as a bolt having a UHMWPE cap, a socket having an UHMWPE jacket, and an UHMWPE product having a reinforcing member encapsulated within the product. The method for making UHMWPE articles uses a compression molding machine equipped with a male and female mold assembly. The female mold has at least one cavity having the size and shape of the completed UHMWPE article. The male mold has a plunger that fits into the cavity to apply substantial pressure to particulate or powdered UHMWPE located in the cavity when the female mold is moved relative to the male mold. The female cavity is open to a bolt retaining opening used to position the head of the bolt in the cavity. The particulate UHMWPE is subjected to a large compression force to purge the particulate UHMWPE of air and force UHMWPE under the head of the bolt and densified the UHMWPE around the head of the bolt. An alternative female mold has a boss on the bottom wall of the cavity for accommodating the socket of a socket head cap screw. A collar of UHMWPE is molecularly bonded to the head of the cap screw. A reinforcing member, such as a metal plate, fabric, or glass fibers, is incorporated in the UHMWPE by compressing particulate UHMWPE about the reinforcing member and molecularly bonding the UHMWPE to the reinforcing member. The compressed particulate UHMWPE is heated to a temperature of between 250 and 350 degrees F. to molecularly bond the UHMWPE to the part, such as the head of the bolt, socket, and reinforcing member located in the mold cavity. The male and female molds are heated. The heat is transferred by the mold to the compressed UHMWPE and sustained for a time to ensure that the UHMWPE is totally sintered and molecularly bonded to the article located in the mold. The pressure on the heated UHMWPE is maintained during the heating process. Upon completion of the heating process the mold and UHMWPE is allowed to cool. A cooling liquid is used to enhance the cooling rate of the UHMWPE and article. The pressure on the UHMWPE is maintained during the cooling duration. The cooled combined UHMWPE and article is removed from the female mold by releasing the pressure on the UHMWPE and retracting the cavity from the plunger of the male mold. Air under pressure may be used to separate the UHMWPE and article from the female mold.
The invention includes the combined UHMWPE and article, such as the head of a bolt, a socket, a cap screw, and a reinforcing member. The bolt has a head enclosed within a UHMWPE cap or body with the UHMWPE molecularly bonded to the top, side and bottom surfaces of the head of the bolt. The socket is a hand tool having a cylindrical body. A UHMWPE jacket is molecularly bonded to the outside of the cylindrical body. The cap screw has a head having a socket in its outer end and an outer surface. A collar of UHMWPE is molecularly bonded to the outer surface and end of the head. The reinforcing member, such as a metal plate, metal member, fabric, fiber member or a glass fibers, is located within a body of UHMWPE with the UHMWPE bonded to the reinforcing member to increase the strength of the body of UHMWPE.